1. Field of the Invention
The present invention relates to a method of obtaining close contact between a master device and a replicate device and, more particularly, to such a method for use in applications such as optical or magnetic contact printing.
2. Description Relative to the Prior Art
In optical contact printing, a master device containing imagery is placed in contact with a replicate device upon which it is desired to replicate the imagery. One common form of master device is a photomask comprising a piece of processed photographic film containing an image pattern. A suitable replicate device could be a piece of unexposed photographic film. Maintaining close contact between the master and the replicate devices during the contact printing process is critical if imagery containing small dots, narrow lines or other high frequency information is to be replicated. Several methods have been generally employed to maintain intimate contact between the master and replicate devices.
One such method involves the use of mechanically applied pressure. In optical contact printing, for example, an image bearing layer of a master device is placed in contact with a photosensitive layer of a replicate device to form a master-replicate sandwich. The master-replicate sandwich is placed on an exposure platen (e.g., a flat piece of translucent glass illuminated from behind by a light source) with the master device against the platen. Pressure is mechanically applied to the master-replicate sandwich by pressing down on the replicate device with a soft pad so as to apply pressure without damaging the master-replicate sandwich. The master and replicate devices are thus pressed into contact and a contact print exposure can be made through the exposure platen.
Because methods such as the one described above in which a master device and a replicate device are mechanically squeezed together are not well suited for maintaining intimate contact between the master and replicate devices, other methods of contact printing have been employed. A form of contact printing often used when close contact is critical (i.e., when printing imagery containing higher frequency information) is a form commonly referred to as vacuum contact printing. In this method, a vacuum pump is used to evacuate air from between the master and replicate devices, thus leaving a vacuum therebetween which has the effect of tightly pressing the master and replicate devices together. It has been found that vacuum contact printing works well and that intimate contact can be maintained, at least over relatively small areas. Over larger areas, however, it is difficult to satisfactorily evacuate the air from between the master and replicate devices with the result that air pockets remain which prevent intimate contact. The quality of the printed image thus tends to be somewhat uneven--of excellent quality in those areas of the replicate device where intimate contact with the master device was maintained, but of poor quality in those areas of the replicate device where pockets of trapped air resulted in a complete loss of contact.
There are applications other than optical contact printing wherein it is desired to replicate imagery from a master device to a replicate device. As one example, in magnetic contact printing a master device of high coercivity having magnetic images thereon is placed in contact with a magnetic replicate device of low coercivity such as a blank piece of magnetic recording tape. The master-replicate sandwich is subjected to a magnetic field of insufficient strength to erase the master device but of sufficient strength to record on the replicate device. Under the influence of the applied magnetic field, the magnetic imagery on the master device is replicated on the replicate device. The quality of such image replication depends upon the contact maintained between the master and replicate devices. Like optical contact printing, magnetic contact printing depends upon intimate contact for optimum results.
In the case where the magnetic materials comprise strips of magnetic recording tape, the master and replicate devices are often contacted by squeezing them together between a pair of pinch rollers or other similar mechanism. More recently, however, it has become an increasingly common practice to record image information on magnetic discs or other large area memory devices. Maintaining close contact over the relatively large areas associated with such magnetic devices poses problems similar to those encountered in the optical contact printing of large area imagery, discussed above.